"Were secondary duties aircraft in WW2 painted yellow underside and camo uppers? Was THAT a regulation?"
No. They were supposed to conform with the relevant regulations for their type, role and organisation.
Training aircraft were supposed to have yellow undersides and received the disruptive scheme on upper surfaces in 1938 along with everything else. Aircraft at OTUs were camouflaged as per whatever command they belonged to as were those of the Air Fighting Development Unit, Fighter Interception Unit and, while it existed, No. 2 School of Army Co-operation Command. .
Experimental and prototype aircraft, including private venture aircraft, were to have yellow undersides, though operational aircraft temporarily on the strength of experimental establishments were allowed to retain their 'normal colourings'.
Ambulance aircraft - white under surfaces.
Target towing aircraft - black inclined stripes on a yellow background. Black stripes 3 feet wide with a six foot yellow spacing.
The last one came with a helpful diagram:

In all cases the demarcation between upper and lower colours was determined by one of two 'patterns'. The detail is beyond the scope of this reply, but 'Pattern No. 1' was a low demarcation, 'Pattern No. 2' much higher. Which was to be used was specified in the instructions for each class of aircraft.
Aircraft were supposed to be finished in accordance with the relevant Air Ministry Orders. However, there was a war on, supplies of the relevant materiel, particularly to overseas commands for aircraft outside the metropolitan air force were inconsistent and the further away from London the less likely things were to be done on time, or even at all.
Malta is not that far away, but look at the can 'o' worms the colours on the Spitfires delivered there can be.
No. They were supposed to conform with the relevant regulations for their type, role and organisation.
Training aircraft were supposed to have yellow undersides and received the disruptive scheme on upper surfaces in 1938 along with everything else. Aircraft at OTUs were camouflaged as per whatever command they belonged to as were those of the Air Fighting Development Unit, Fighter Interception Unit and, while it existed, No. 2 School of Army Co-operation Command. .
Experimental and prototype aircraft, including private venture aircraft, were to have yellow undersides, though operational aircraft temporarily on the strength of experimental establishments were allowed to retain their 'normal colourings'.
Ambulance aircraft - white under surfaces.
Target towing aircraft - black inclined stripes on a yellow background. Black stripes 3 feet wide with a six foot yellow spacing.
The last one came with a helpful diagram:
In all cases the demarcation between upper and lower colours was determined by one of two 'patterns'. The detail is beyond the scope of this reply, but 'Pattern No. 1' was a low demarcation, 'Pattern No. 2' much higher. Which was to be used was specified in the instructions for each class of aircraft.
Aircraft were supposed to be finished in accordance with the relevant Air Ministry Orders. However, there was a war on, supplies of the relevant materiel, particularly to overseas commands for aircraft outside the metropolitan air force were inconsistent and the further away from London the less likely things were to be done on time, or even at all.
Malta is not that far away, but look at the can 'o' worms the colours on the Spitfires delivered there can be.
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